QUANTUM INTERFERENCE IN A MESOSCOPIC SUPERCONDUCTING LOOP

THE classical superconducting quantum interference device (SQUID) is based on the Josephson effect1, and usually consists of a macroscopic superconducting loop with two artificial weak links (Josephson junctions) through which the supercurrent passes by quantum-mechanical tunnelling. Fink et al.2 proposed a new type of SQUID based on a homogeneous mesoscopic superconducting loop, in which interference between the supercurrents passing through the two halves of the ring results in a critical current that varies with the applied magnetic field in an oscillatory manner. Here we describe the experimental observation of these oscillations in a mesoscopic superconducting aluminum loop without artificial weak links. In this new type of quantum interferometer, 'weak-link' regions with a strongly reduced superconducting order parameter appear periodically at half-integer magnetic flux quanta owing to the interplay between the shielding and transport currents in the loop.